The results of rigged animal tests that have plunged the German car industry into renewed turmoil were suppressed because they showed the opposite of what car-makers wanted, it has emerged.

Monkeys were forced to breathe diesel exhaust fumes from a VW Beetle for several hours in an attempt to prove they were not toxic in a study funded by Volkswagen, BMW and Daimler, the company that makes Mercedes.

But it has now emerged that the experiment found the exhaust was more harmful than fumes from a 20-year-old diesel car — despite the fact that the Beetle was fitted with software designed to lower emissions during tests.

The car-makers allegedly refused to pay for the tests, which were conducted in the US, after the results weren’t what they were expecting.